Distributed amplifier with low noise



March-31, 1964 R. E. STURM ETAL DIS'IRIBUTED AMPLIFIER WITH Low NOISE 3Sheets-Sheet 1 Original Filed June 2,` 1954 March 31, 1964 R. E. STURMETAL DISIRIBUTED AMPLIFIER WITH Low NOISE 3 Sheets-Sheet 2 OriginalFi'led June 2, 1954 zNvENToRs Mgr/L5? Zw/'w dan.)

BY Sap/o aan Sapc'm ATTORNEYS March 31, 1964 R. E. STURM ETAL 3,127,568

DISTRIBUTED AMPLIFIER WITH Low NOISE Original Filed June 2, 1954 3Sheets-Sheet 5 BY 'S/{aoim alza.l S/apim ATTORNEYS United States PatentO 3,127,568 DISTRIBUTED AMPLIFHER WITH LOW NOISE Ralph E. Sturm,Pilresville, and Russell H. Morgan, Baltimore, Md., assignors to TheBendix Corporation, a corporation of Delaware Original application JuneZ, 1954, Ser. No. 433,955, now Patent No. 2,399,494, dated Aug. 11,1959. Divided and this application July 16, 1959, Ser. No. 827,511

12 Claims. (Cl. E60-54) This invention relates to novel circuits, andmore particularly to circuits which are useful in the translation ofintelligence having a low signal-to-noise ratio, especially circuitsemployed for or in connection with the augmentation of a low levelsignal without emphasizing noise. An exemplary use of the invention isin the field of iiuoroscopic screen intensification. This application isa division of Serial No. 433,955, filed June 2, 1954, in the names ofthe present applicants and entitled System for the Translation ofIntelligence at Low Signal-to- Noise Ratios, now Patent No. 2,899,494,issued August 1l, 1959.

In the aforementioned parent application is described and claimed asystem for translating low level signals in the presence of electricalnoise of the same order of magnitude. As therein set forth, the systemmay comprise a closed link television chain intensifier having suitablecamera, amplifier, kinescope, and auxiliary circuits. Prior televisionintensiiiers were limited by their noise level, and use of the bestcomponents available failed to produce any significant improvement inthe performance of the conventional television intensifier.

it was generally assumed in the prior art that the noise observed on theviewing screen of a conventional television system is the manifestationof random noise existing in nature, such as orthicon beam noise, shoteffect, thermal agitation or resistor noise, etc., and that since suchnoise is inherent in the system, its eiiects can not be eliminated.Contrary to this generally accepted view, the applicants found that thenoise present on the television screen is far in excess of that whichwould be predicted from classical noise theory, and moreover, thatinstead of being random, such noise has a definite spectralcharacteristic, which produces much greater deterioration of the picturethan would be expected from theory. More specifically, it was discoveredthat the excessive noise present on the viewing screen is caused byshock excitation of underdarnped modes of vibration of the circuitry bynoise occurring in the input, which results in the augmentation inamplitude and compression into a narrow frequency band of the randomnoise existing in nature.

As explained in the aforementioned parent application, the applicantsdiscovered that the solution to the problems set forth above, that is,the elimination of circuit oscillation in response to random noiseexcitation and the elimination of the accompanying amplitudeaccentuation and frequency compression of random noise, lies in the useof a system having circuits whose vibration modes within the entireoperating range of frequencies are at least critically damped. Thepresent application is directed to these circuits.

It is accordingly a principal object of the invention to provide novelcircuits for translating intelligence having a low signal-to-noiseratio.

A further object of the invention is to provide novel amplifiercircuits.

An additional object of the invention is to provide a novel scheme forintroducing blanking signals into a television system or the like.

A still further object of the invention is to provide a novel cathodefollower circuit or the like and method of operating the same.

The foregoing and other objects, features, and advantages of theinvention and the manner in which the same are accomplished will becomemore apparent upon consideration of the following detailed descriptionof the invention when taken in conjunction with the accompanyingdrawings which illustrate preferred and exemplary embodiments, andwherein:

FIGURE l is a diagrammatic showing of the overall system of the parentapplication employed in a screen intensifier;

FIGURE 2 is a circuit diagram of one section of an amplifier constructedaccording to the invention;

FIGURE 3 is a circuit diagram of a device of the invention forintroducing blanlring signals into the intensilier;

FIGURE 4 is a graphic illustration of the operation of a conventionalcathode follower under conditions to be described hereinafter;

FIGURE 5 is a circuit diagram of a modified device of the invention forintroducing blanking signals; and

FIGURE 6 is a block diagram showing a preferred arrangement of theamplifier sections of the invention.

In FIGURE l the general scheme of the: invention described in the parentapplication is shown. The invention of that application is illustratedas applied to an X-ray intensification system including an X-ray control1t) for controlling and operating an Y-ray tube 12, which projects abeam of X-rays onto a fluorescent screen 16 through a subject 14. A grid15 may be placed before fluorescent screen 16 to reduce scatter. Theimage produced on the iuorescent screeen 16 under the action of theX-rays is focused by an optical system represented by lens 18 onto thelight sensitive element of a pick-up tube 2i). This tube may be an imageorthicon of the type conventionally employed in television practice.Block 2li may also include the necessary sweep circuits and controls forthe image orthicon tube.

The electrical signals corresponding to the image produced on the lightsensitive element of the pick-up tube are applied to a critically dampedamplifier 22 which will be described in more detail hereinafter. Block22 may include controls to set the contrast of the picture produced onthe fluorescent screen of a kinescope 24, to which is applied theamplied signals from the critically damped amplifier 22. A pulse formerand Shaper 26 supplies the necessary pulses to initiate the operation ofthe sweep circuits associated with the image orthicon and the kinescopeat precisely the same time so that the picture which is broken up intosmall increments bythe image orthicon will be reassembled into exactlythe same increments at a greater brightness by the kinescope 24. Ifdesired, the screen of the kinescope 24 may be photographed to provide apermanent record of the observation. A suitable power supply (not shown)furnishes all of the power requirements for the intensifying unit. Asset forth in the parent application it was discovered that the entiresystem through which the video signal passes must be at least criticallydamped if the accentuation of noise is to be eliminated.

In ordinary television practice, ie., a 525 line interlaced scanningsystem, amplifiers must be capable of maintaining good amplitude andphase response over a frequency range of the order of 60 cycles persecond up to approximately 4 megacycles per second. In a system such asthat illustrated in FIGURE l, the useful frequency range may be fromabout 50 cycles to 15 megacycles per second. It kis well known thatordinary amplifier tubes in resistance-capacitance coupling will notcover the frequency range required in conventional television practicewhile producing optimum gain without the inclusion of special peakingcircuits which compensate for the input capacitance of the tubes as wellas the capacitance associated with the layout, wiring, and thecomponents. Both shunt and series peaking as well as a combination ofthe two are employed. Peaking is usually accomplished by adding therequired amount of inductance to correct for amplitude and phase angledistortion. In general, peaking renders the circuit oscillatory in oneor more modes of vibration, and overshoot and subsequent oscillationoccur because of the oscillatory condition. In order to correct forphase and amplitude distortion without employing an excessive number oftubes, which in turn would increase the noise of the system, it isnecessary that the conventional circuits be oscillatory. If the standardpeaked amplifier circuit were modified so that the gain of each stagewere low enough to prevent oscillation, many additional stages would berequired to produce the necessary over-all gain, and ultimately thenoise introduced by the input tubes and their parameters would defeatthe purpose.

The critically damped amplifier which forms a part of the presentinvention utilizes the long line or distributed constant principle. Thisgeneral principle of amplifier design is, of course, not new to the art,since at the higher frequencies Where the ordinary shunt or seriespeaking is not effective in correcting phase and arnplitude distortion,amplifiers built on the theory that each tube is a part of thedistributed capacitance of a long line have been employed to obtain wideband-width and high gain. Such amplifiers operate satisfactorily up tofrequencies of several hundred megacycles when not limited by circuiteffects outside the tubes. However, no reference is found in the priorart to the adaptation of a line amplier to prevent the emphasis of noisein the translation of intelligence having a very low signalto-noiseratio. In fact the large number of tubes required by such an amplifierwould lead one to believe that line amplifiers are unsuitable for suchuse, because of the increased noise which would be expected from theemployment of so many tubes.

FIGURE 2 illustrates one section of the amplifier of the invention.Actually the complete amplifier may comprise several sections similar tothat illustrated, each section connected to the previous one in cascade,as shown in FIGURE 6. Each section comprises a plurality of drivingdevices exemplified by the tubes 28 to 40. In this particularembodiment, seven pentodes, such as the 6CB6, are employed. The controlgrids of the respective tubes are connected to a grid line 46 comprisinga series of coils 48 to 62 and condensers 108 to 120'. Successive coilsmay be wound in opposition to reduce the mutual .coupling betweenadjacent coils to the lowest level possible, but this is not essential.The grid line is terminated at its respective ends in its surgeimpedance by resistors 64, `66, respectively, and the small paddingcondensers 108 to 120 are employed to correct for variations in tubecapacitances and to bring the surge impedance of each section of theline to the `correct value.V By proper adjustment the grid line may bemade substantially reflectionless.

The anodes of the respective tubes are connected to a plate line l68comprising coils 70 to y84, which also may be wound successively inopposition. Here again padding condensers 94 to y1416 are employed toadjust the respective sections of the line to the correct surgeimpedance. The plate line may be terminated at one end by a plurality ofresistors 86, 88, r9i). The other end need not be terminated in itscharacteristic impedance, and this arrangement substantially doubles thegain, as lis known in the art. As will become more evident hereinafter,reflections produced by failure to terminate one end of the plate linein its surge impedance will not greatly affect the operation of thecircuit where pentodes are employed, because of the fact that beyond acertain voltage the plate voltage of a pentode does not substantiallydetermine its plate current. The terminating resisters 86, y83, 911 onthe plate line may be quite critical, since in this application they arerequired to have about 13 watts dissipation with negligible inductance.Ordinary non-inductive resistors of the wire-wound type may not besatisfactory, but the type R33 non-inductive resistor produced by theCorning Glass Company, or its equivalent, may be employedsatisfactorily.

An input driving device illustrated by pentode 42, which may be a `6AH6tube, has its anode connected to inductance 48 of the grid line and itscontrol grid connected to input terminal 156 through a coupling networkincluding coupling condenser 158 and grid return resistor 161i. Asuitable cathode load resistor 1318 is provided. It will be noted thatresistor 138 in series with a resistor 136 form a cathode load for theline tubes 23 to 4t).Y These resistors are connected to the respectivecathodes of the line tubes through lead 134 and are lay-passed to groundthrough condensers 140, 142. The flow of plate current of the line tubesthrough resistors 136, 138 produces a small positive feed back whichresults in better low frequency response. The feed back is operativeonly at the extremely low frequencies, since the higher frequencysignals are shunted by condensers 141i, 142. Control over the feed backis accordingly obtained by selection of the values of condensers 141),142. Cathode load resistor 138` may be shunted by a small condenser (notshown) to provide high frequency peaking and phase shift, if desired.

The passage of the line tube plate currents through resistors 136, 138is also utilized to provide well regulated voltages for the grid line 46and to decouple the grid line from the power supply. In operation, eachline tube may have approximately 12 milliamperes flowing through it, andseven line tubes will give a total current of approximately 84milliamperes. The resistance of resistor 136 may be approximately athousand ohms, and the resistance of resistor 138 may have a relativelylow value. The combined line tube plate .currents passing through theseresistors in seriesproduces a regulated potential of approximately 84volts at the cathodes of the line tubes, and this potential is appliedto the plate of input tube 42 through terminating resistors 64, 66.Condensers 140, 142 also provide a lter for the input tube platepotential.

The screen grid of tube 42 is fed from the B supply at terminal 122through variable resistor 146 and fixed resistor 148, and is by-passedto ground by condensers 152, 154. Resistor 146 may be employm -tocontrol the plate current of tube 42. Since the D.C. plate current ofthe input tube flows through resistors 64, 66, which lie in the lcontrolgrid to cathode path of tubes 28 to 40, resistor 146 may also beemployed to control the grid bias on tubes 28 to 40.

An output translating device, which has been illustrated as a triodetube 44 connected as a cathode follower, is coupled to that end of theplate line which is not terminated in its characteristic impedance, by aphase corrective network 171, which may comprise variable inductance170, capacitor 174 and resistances 172, 176. A coupling condenser 166, agrid return resistor 168, and a cathode load resistor 164 are providedfor the output tube. The output terminal 92 is connected to the cathodeof the tube.

The B supply voltage fed to each of the `amplifier tubes from terminal122 should be very carefully regulated. Since shock excitation as Wellas standing waves at high frequencies may occur on the lead wires fromthe B supply, a decoupling network consisting of a resistor 124 andcondenser 126 is inserted tc decouple the amplifier from the powersupply and to prevent these effects. The value of resistor 124 is madelarge enough so that the inductance of the line feeding the amplifiertogether with the distributed capacitance will not oscillate when shockexcited.

The screen grids of tubes 28 to 40 are fed from the B supply throughdropping resistor 12.8 and condensers 130,

132, which form a iilter network. A plurality of resistors 134a throughllda is inserted in series which the respective screen grids of the linetubes. These resistors are employed to prevent spurious oscillations dueto the inductance and capacitance of the lines feeding the screen gridsof the particular tubes, that is, they are employed to ensure at leastcritical damping. In practice, it may be necessary to insert smallresistors (such as resistors 161, M2, i165 associated with tubes 42, 4d)in series with the grids and plates of all tubes except the line tubesper se to counteract any tendency toward oscillation of the inductanceof the leads taken in conjunction with the distributed and tubecapacitances. In this connection it should be noted that for optimumresults it may, in some instances, be necessary to insert smallresistors in the lament leads of the tubes and in the lines between theamplifier sections. Damping resistors are inserted wherever anoscillatory condition of inductance and capacitance would exist in theirabsence. These resistors may be of some convenient value, such as of theorder of 47 ohms.

Where large condens/ers, such as electrolytics, are required in thecircuit illustrated, they must be shunted by smaller condensers in orderto ensure the desired high frequency response. lt is Well known that anelectrolytic capacitor, for instance one having a capacitance of ahundred microfarads, is not satisfactory for use at high frequencies. Toovercome this each of the large condensers is shunted with a smallercondenser, such as a .0l microfarad. Thus in FIGURE 2 condensers ltl,140 and d, which may be large electrolytic capacitors, are shnnted bysmaller condensers 132, 142 and 152, respectively.

Considering the operation of the circuit illustrated in FIGURE 2, a lowlevel signal at input terminal ld is applied to the control grid ofinput tube 42 through the coupling network 153, let?. For purposes ofillustration it is assumed that the input signal is a square wave withpositive polarity. This wave will increase the current in the inputtube, which will, by means of resistors 64, 66, produce a decrease inthe voltage at the plate of tube 413. For extremely low frequencies, theinductances 4S through 62 have practically no effect, and resistors 64,66, are essentially in parallel. However, at high frequencies, theseinductances do have substantial eects, and consequently, it can be seenthat the input signal produces a negative square wave which proceedsdown the grid line 46 toward resistors da, d5. It is evident from linetheory that a line has a finite propagation time depending upon itsparameters, that is, it takes a finite length of time for a voltage waveto move down the line. The propagation time may be controlled by theparameters R, C and L including parallel conductances (not shown) ineach section of the line. The padding condensers N3 through 1Z0 may beadjusted to compensate for varying input capacitance of the tubes sothat the propagation constant is the same for each section of the line.The propagation constant of the line as a whole will, therefore, belinear, and the negative signal at the input of the line will movesmoothy and linearly toward resistors 64, 66. A wave which is incidentupon either of resistors dfi, 65 will be completely absorbed, since theline is terminated at each end in its surge impedance.

In the plate line d3 the padding condensers 9d through lila maysimilarly be adjusted to ensure a linear propagation constant for theplate line as a whole. While the termination comprising resistors 36,d8, 90 of the plate line may have a different value from the terminatingresistors of the grid line, the propagation constants for the two linesmay nevertheless be made exactly the same. Assuming this to be the case,when the negative square wave produced at the plate of input tube 42reaches the control grid of tube 28, it produces `in the plate circuitof this tube a positive pulse which is an amplified inversion of thepulse incident upon the control grid. Current through tube 2S, as wellas plate current for all of the other line tubes, must flow through thetermination 86, S8, 9d. Thus, the pulse produced at the plate of tube 28will start down the plate line toward its ends. Since the propagationconstants of the plate and grid lines are identical, as the negativepulse moves down the grid lines, the positive pulse will move down theplate line, and each time the negative pulse is incident on the grid ofa line tube, the plate of such tube will add a positive pulse to the onealready existing from the previous tube. The signal will be built up asthough all seven tubes had been connected in parallel and all of theirtransconductances had been operating on the load resistance 86, 8S, 90.

A grid line signal is completely absorbed in the terminating resistors64, 65. However, in the plate line, since the far end is not terminatedin its characteristic impedance the signal will be reflected dependingon the type of termination. Consequently, a reiiected signal will startback down the plate line passing each of the tubes in turn and finallyarriving at the terminating resistance 86, 88, 9?. Here the reflectedsignal will be completed absorbed, because the line at this point isterminated in its surge impedance. As indicated previously, thereiiected signal will not aifect the operation of the circuit, becausethe plate current of a pentode is substantially independent of its platepotential beyond a certain potential.

The useful signal on the plate line passes through the phase correctivenetwork 171 and is applied to the control grid of cathode follower 44.The signal incident on the grid of the cathode follower produces asignal at the cathode which can be fed to the next amplier section froma substantially low impedance source. This tube acts as an impedancechanger and a decoupling tube, so that whatever is connected to theoutput of the amplifier section will not have a substantial effect onthe characteristics of the plate line of this particular section.

Three sections identical to that illustrated in FIGURE 2, with theexception of special input and output connections to facilitateintroduction of blanking signals, etc. may be connected in tandem asshown in FIG. 6, producing a maximum gain of from 400,000 to 1,000,000.Such an amplifier system has been tested in a closed link televisionchain of the type illustrated in FIGURE l employing a standard imageorthicon tube of the 5 820 type, and it has been found that withcritical damping the noise is reduced by a factor of the order of 20 to80 times over a system using a standard shunt-series peaked amplifier.Good pictures were obtained on the screen of the kinescope down to lightlevels of 10-4 millilarnberts, and in particular it was observed at thislevel that the small amount of noise that remained was entirely randomand did not interfere with the resolution of the picture nearly as muchas did the previous periodic noise at the higher light levels.

The fact that the critically damped amplifier of the in- Vention doesnot accentuate noise also implies, and it is proved in actual practice,that any signal, and in particular the square wave variety Where therise time is extremely fast, will be reproduced essentially faithfullywithout overshoot or ringing. In conventional amplifiers, such as theshunt and series peaked type, this is not the case. This means that muchbetter resolution may be obtained through use of the invention.

Since the power distribution of random noise is equal in any givenfrequency interval of the spectrum, it is evidently desirable to limitthe spectrum of the system in order to reduce the noise to the lowestpossible value. This may be done by limiting both the top and bottom ofthe signal pass band of the system, preferably at its input. It can beshown that a system may be shock excited by a signal, such as a spuriousoscillation, which is entirely outside the pass band of the system. Ithas been found that when one controls the phase and amplitudecharacteristics of the limits of the pass band so as to produce thenarrowest pass band that is necessary to transmit the intelligence andat the same time to prevent oscillations, the lowest possible noiselevel is obtained.

The term critically damped as employed in the speciiication and claimsdescribes the condition of a circuit in which the ability of the circuitto oscillate just ceases to exist. For example, in a simple seriescircuit having inductance L, capacitance C, and resistance R, criticaldamping exists when the solution to the differential equation for thecurrent in the circuit is such that the discriminantis equal to zero, orwhere If the left-hand term of this equation is greater than theright-hand term, the circuit is over-damped. In both instances thecircuit is non-osciliatory, but if the lefthand term is smaller than theright-hand term, the circuit is oscillatory. As employed in thespecification and claims the term at least critically damped refers to acircuit which is either critically damped or over-damped, i.e.,non-oscillatory.

The ideal condition of exact critical damping is dithcult to achieve,and in practice the condition is approached as` a limit from the regionof over-damping. The amplier must be at least critically damped throughits entire operating range, Which includes its pass band and bandskirts. Auxiliary circuits of the amplifier through which the signaldoes not pass but which may introduce spurious oscillations, such aspower supply leads, leads for inserting blanking signals, etc., shouldof course be at least critically damped to prevent noise enhancement,but may be substantially over-damped without detracting from fidelity ofreproduction.

The output of the iinal amplifier section may be required to drive akinescope as indicated in FIGURE 1. Good design requires that theamplifier be able to drive the kinescope from cut-olf to cut-off eventhough this may not actually be done in practice. For the type ofkinescope employed in the illustrative system at least a 30 volt signalwould be required to accomplish this. In driving the final cathodefollower through a full 30 volts it was noted initially that the lowfrequencies were handled very well with little or no distortion;however, the higher frequency signals which were impressed on the inputwere notably distorted. When square waves with extremely short rise timeWere fed through the circuit, there was a noticeable curving off as thesquare portion of the curve rose, that is, high frequency cut-off orhigh frequency attenuation was noted. It had been assumed in the artthat conventional cathode followers would handle signals up tofrequencies at which transit time effects become important. It was foundthat the capacitance between the cathode and the filament of the cathodefollower as well as the capacitance of the associated parts of thecircuits delayed the rise time or fall time of the signal applied to thecathode so that it did not follow the grid instantaneously. Underordinary conditions such a phase lag could be corrected in the circuitif that were the only effect. However, the lag of the cathode withrespect to the grid causes the grid to draw current, upsetting all ofthe relationships in the circuit and consequently causing a badlydeformed wave in the output. It was discovered that this effect may beovercome by arranging conditions so that the tube has a quiescent biasbetween cathode and grid which is always equal to or larger than thesignal applied to the grid.

FIGURE 4 illustrates the phenomenon discussed above. It can be seen thatwhen a square wave is applied to the grid of the cathode follower, thecathode voltage does not rise at the same rate and at time t1, forexample, the grid may be positive relative to the cathode by better than25 volts. The conditions at time t2 indicate that the maximum positivegrid-cathode voltage may reach 50 volts in the example given. The resultis a badly distorted output signal. The quiescent grid-cathode voltagemust, therefore, be chosen so that the grid is at least 50 voltsnegative with respect to the cathode in order to eliminate thephenomenon discussed. This may be accomplished by choosing the tube,plate voltage and cathode load, so that the quiescent current throughthe cathode load is suicient to bias the cathode at least 50 voltspositive with respect to the grid, for the example given. The graphmakes use of linear curves for simplicity, but in practice these curvesare exponential.

In the three-section amplifier of FIG. 6, each section isdirect-coupled, but from section to section resistancecapacitancecoupling is employed. This allows the convenient introduction ofblanking and shading signals, which are preferably not applied directlyto the line tube stages. FIGURE 3 illustrates a unique way ofintroducing the blanking signal. This signal is generally employed tocut ofi the beam of the kinescope during the return trace of the cathoderay so that the latter does not interfere with the picture, and in theparticular system disclosed it is also utilized to set the D.C. blacklevel in association with the circuits that follow so that contrastcontrol is obtained in the final picture. In the circuit of FIGURE 3 thevideo input signal at terminal 262 is applied through a networkcomprising coupling condenser and grid return resistor 182 to thecontrol grid of a triode 178A. The blank signal, which may be fed from alow impedance cathode follower source, is applied from terminal 2da tothe control grid of a triode 178B through a coupling network comprisingcondenser 184 and grid return resistor H56. As indicated in the drawing,tubes 178A and USB may be constituted by two sections of a dual triodetube. The triodes are provided with a common cathode resistor 188 andare connected to a source of B supply 1% through a decoupling networkcomprising resistor 194 and condensers 19%, 2%. The latter condenser isof relatively small value and shunts the larger condenser 198 (which maybe electrolytic) for the higher frequencies, in the manner set forthpreviously. Resistor 192 is a plate dropping or load resistor for tube178B, while resistors 161, 183, are small resistors employed toeliminate parasitic oscillations and to ensure critical damping. Thevideo input signal on the control grid of tube HSA is coupled from thecathode of the latter to the cathode of tube 178B. So far as the signalapplied to the cathode of tube 178B is concerned, this tube operates asa grounded grid amplifier, which allows operation at a higher frequency,because the input capacitance is quite low. The control grid of tube178B serves as a mixer grid to which the blank signal is fed, and theoutput is taken on lead 195 from the plate of this tube. The dual triodeI78AB may constitute the output tube corresponding to tube 44 of FIG. 2for the intermediate amplier section of FIG. 6. This substitution isindicated in the drawings and may be accomplished by breaking thecircuit of FIG. 2 at points X and connecting in place of tube 4.4, etc.,the circuit of FIG. 3 at the points X1. This arrangement operates wellup to and including frequencies of 'l5 megacycles, giving an appreciablegain, depending on the transconductance of the tubes, without employingpeaking devices. Thus, an amplifying stage is provided which may be usedtogether with the line ampli- Iier to obtain additional gain and tosolve the problem of mixing without introducing any deleterious effectson the signal, as would occur with a stage in which peaking wereemployed in order to properly correct for amplitude and phasedistortion. The use of a dual triode allows extremely short cathodeleads and, therefore, minimizes cathode inductance. If separate tubeswere employed, the parameters of the cathode circuits could be adjustedto produce a filtering action, if desired.

Alternatively, blank signals could be introduced by ernploying a secondpentode in parallel with the input tube 42 in FIGURE 2, connecting theplates of the tubes together and utilizing separate screen grid, cathodeand control grid connections. This arrangement is illustrated in FIG. 5,wherein tube 42 and associated components correspond to those shown inFIG. 2; only that portion of FIG. 2 necessary to the description isrepeated. The

anode of parallel tube 210 is connected to the anode of tube 42, and thecathode is connected to ground through a bias network including resistor212 and condensers 214, 216. The screen grid of tube 210 is fed from theB supply through a variable dropping resistor 224 and bypass condensers226, 228. Condensers 216 and 228 may be employed to shunt largercondensers 214, 226, as set forth previously. The blank signal fromterminal 222 is coupled to the control grid of tube 210 throughresistor-condenser network 218, 220. In operation, the bias of tube 210is adjusted so as to prevent large shunting of tube 42, therebypreventing substantial loss in gain for tube 42. It has been found thatsatisfactory operation results if tube 42 carries 80% of the combinedplate current, provided the blank signals are sufficiently strong. Whilethis arrangement makes an excellent mixing system, there is some loss ofthe normal gain of tube 42.

The introduction of a shade signal, indicated in FIG. 6, may beconveniently accomplished by applying the required saw tooth voltage tothe cathode and/ or the control grid of the input tube (corresponding totube 42 in FIG. 2) of the intermediate amplifier section. The need forsuch signals is well known in television practice, and systems forapplying such signals are also well known. Contrast control may beachieved by inserting a gain control potentiometer in the input to theintermediate amplifier and a suitable black level setter in the outputof the output amplier section.

While a preferred embodiment of the invention has been shown anddescribed, it will be apparent to those skilled in the art that changescan be made in this embodiment without departing from the principles andspirit of the invention, the scope of which is defined in the appendedclaims. For example, the principles of the invention may be applied totransistor circuits as well as vacuum tube circuits, and the term tubeas used herein is intended to be generic to any type of electron drivingdevice employed in accordance with the invention. Accordingly, theforegoing embodiment is to be considered illustrative, rather thanrestrictive of the invention, and those modifications which come withinthe meaning and range of equivalency of the claims are to be includedtherein.

The invention claimed is:

1. A line amplifier comprising an anode transmission line, a gridtransmission line, a plurality of line tubes having anodes connected insequence to said anode line and control grids connected in sequence tosaid grid line, a source of anode potential having one terminalconnected to said anode line, terminating impedance means for said linetubes, an input tube having its anode connected to one end of said gridline and having a cathode load connected between its cathode and anotherterminal of said source, means connecting the cathodes of said linetubes to the cathode of said input tube and through said cathode load tosaid other terminal, the anode supply path for said input tube includingsaid grid line terminating means and said line tubes, said meansconnecting the cathodes of the line tubes to the cathode of the inputtube comprising a low pass iilter, and said cathode load for the inputtube constituting a cathode load for said line tubes.

2. The amplilier of claim 1, including means for manually adjusting theoutput current of said input tube, whereby the grid bias of said linestubes may be adjusted.

3. The ampliiier of claim l, said input tube having a screen gridconnected to an adjustable source of potential.

4. The arnplifier of claim l, said input tube having another tubeconnected in parallel therewith, said other tube having a controlelement adapted for connection to a source of blanking signals.

5. The amplifier of claim 4, said other tube having a screen gridconnected to a source of adjustable potential.

6. A line amplifier comprising an anode transmission line, a gridtransmission line, a plurality of line tubes having anodes connected insequence to said anode line and control grids connected in sequence tosaid grid line, a source of anode potential having one terminalconnected to said anode line, terminating impedance means for said gridline connecting said grid line to the cathodes of said line tubes, aninput tube having an anode connected to one end of said grid line andhaving a cathode load connected between its cathode and another terminalof said source, means connecting the cathodes of said line tubes to thecathode of said input tube and through said cathode load to said otherterminal, an output cathode follower connected to said anode line, saidline tubes having screen grids connected to said source of anodepotential through damping resistors, said input tube having a controlgrid connected to an input terminal through a damping resistor, and saidcathode follower comprising a tube having an anode connected to saidsource of anode potential through a damping resistor and having acontrol grid connected to said anode line through a damping resistor,the entire amplifier being at least critically damped.

7. A line amplifier comprising an anode transmission line, a gridtransmission line, a plurality of line tubes having anodes connected insequence to said anode line and control grids connected in sequence tosaid grid line, a source of anode potential connected to said anodeline, terminating impedance means connecting the grid line to thecathodes of said line tubes, and a cathode follower including a tubewith a grid connected to said anode line through a phase-correctingnetwork.

8. The amplifier of claim 7, further comprising an additional tube withits cathode connected to the cathode of the cathode follower tube andits anode connected to a source of anode potential through an anodeload, said additional tube having a grid adapted to be connected to alow impedance source of blanking signals.

9. The ampliiier of claim 8, said cathode follower tube having dampingresistors in series with its control grid and its anode, and saidadditional tube having a damping resistor in series with its grid, theentire amplifier being at least critically damped.

10. The amplier of claim 8, said additional tube having an outputconnection from its anode.

l1. The amplifier of claim 7, said cathode follower having means forapplying a positive bias to its cathode at least as large as the maximumpositive amplitude of the signals applied to its grid.

12. A line amplifier comprising an anode transmission line, a gridtransmission line, a plurality of line tubes each having an anode, acathode, a control grid, and a screen grid, said anodes being connectedin seq uence to said anode line and said control grids being connectedin sesequence to said grid line, said grid line having means at each endthereof for terminating said grid line in its characteristic impedance,said cathodes being connected to each end of said grid line through saidimpedance means, said anode line having means at one end thereof forterminating said anode line in its characteristic impedance, a source ofB-ipotential, means including a damping resistance for connecting saidsource to said one end of said anode line through its terminatingimpedance means, means including a plurality of damping resistances forconnecting the respective screen grids to said source. an input tubehaving a control grid, a cathode and an anode, an input terminal, meansincluding a damping resistance for connecting said input terminal tosaid control grid of said input tube, means connecting the anode of saidinput tube to one end of said grid line, means connecting the cathode ofsaid input tube and the cathodes of said line tubes to a point ofreference potential, an output tube having a control grid, a cathode,and an anode, means including a damping resistance for connecting theother end of said anode line to the control grid of said output tube,means including a damping resistance for connecting the anode of saidoutput tube to said source, and means for connecting the cathode of saidoutput tube to said point, said amplifier, including each of saidconnecting means, being at least critically damped for all vibrationmodes Within the entire operating range of frequencies.

References Cited in the lile of this patent UNITED STATES PATENTS HarrisFeb. 16, 1937 Burnett May 7, 1940 Wheeler June 25, 1940 Webster Nov. 15,1949 Wiegand et al Apr. 22, 1952 Goldstine Dec. 15, 1953 Kelley Feb. 23,1954 Moe Oct. 25, 1955 12 Bradley Jan. 22, 1957 Brown Aug. 5, 1958Diarnbra et al Dec. 2, 1958 Fischer Dec. 2, 1958 Doshey June 23, 1959FOREIGN PATENTS Great Britain June 2, 1936 OTHER REFERENCES 10Scharfrnan article, Distributed Amplifier Covers 10 to 360 Mc.,Electronics, vol. 25, issue 7, July 1952, pp. 113-115, 121-123.

Tuller article, Distributed Amplifiers, IRE Convention Records, 195 3,Part 5, Circuit Theory.

UNITED STATES PATENT oEEICE CERTIFICATE OF CORRECTION Patent No@ 3,l27,58 Mar-ch 3l', 1964 Ralph E Sturm et ale It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

, Column 7, lines 9 to ll, the equation should appear as shown belowinstead of as in the patent:

IL2 LC column 9, line 50, after "for said" insert grid line connectingsaid grid line to the cathodes of Said wm Signed and sealed this 28thday of July 1964:,

(SEAL) Attest:

ESTON G. JOHNSON EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. A LINE AMPLIFIER COMPRISING AN ANODE TRANSMISSION LINE, A GRIDTRANSMISSION LINE, A PLURALITY OF LINE TUBES HAVING ANODES CONNECTEDWITH SEQUENCE TO SAID ANODE LINE AND CONTROL GRIDS CONNECTED IN SEQUENCETO SAID GRID LINE, A SOURCE OF ANODE POTENTIAL HAVING ONE TERMINALCONNECTED TO SAID ANODE LINE, TERMINATING IMPEDANCE MEANS FOR SAID LINETUBES, AN INPUT TUBE HAVING ITS ANODE CONNECTED TO ONE END OF SAID GRIDLINE AND HAVING A CATHODE LOAD CONNECTED BETWEEN ITS CATHODE AND ANOTHERTERMINAL OF SAID SOURCE, MEANS CONNECTING THE CATHODES OF SAID LINETUBES TO THE CATHODE OF SAID INPUT TUBE AND THROUGH SAID CATHODE